Within the railroad industry, railroad switches, crossing frogs, turnouts and the like are spliced to the rail by inserting massive, threaded fasteners through a series of aligned, longitudinally space holes numbering as high as 20 which extend through the rail web and frog, i.e., the rail joint.
As a general rule, all railroad equipment must be designed to interchange with existing track and rolling stock. As the weight of the train, as exemplified by unit trains, as well as the weight of the car continues to increase, the load and service demands placed on crossings, switches, turnouts, etc. have also increased. Specifically, while the rail itself will flex and is designed to flex, the switch must be secured to or spliced into the rail in a rigid, non-yieldable, but removable manner. Because the rail profile is fixed as is, to some extent, the configuration of the switch, the rail joint can only be strengthened to account for the increased service load by increasing the strength and number of fasteners for the rail joint. As the hole number increases, the on site installation of the switches in which hole alignment is achieved by spud wrenches and the like becomes increasingly difficult. Shank tolerance on the bolt becomes more critical. It should be appreciated that bolt diameters range anywhere from 1 to about 13/8" and shank length extends anywhere from 8"to 30". Sledging 20 such bolts through 20 holes aligned in a track bed with a spud is progressively difficult. Conventional, forged bolts have varying tolerances and aggravate the installation problem. Further, the threaded bolt end is simply chamfered at its outer edge. Even though the bolts are now heat treated to have a high tensile strength, sledging the last bolts through the aligned holes can mar or distort the threads. This, in turn, makes application of the nuts difficult, especially so if standard, conventional, hand operated wrenches are used such as when one switch is to be removed and replaced as contrasted to laying a new section of track in which impact-air wrenches may be available.
Until now, the only improvement made by the railroad supply industry to the increased load demands placed on rail joints has been to increase the strength of the bolt by changing its chemistry to obtain improved physical properties. The bolt remains in its "as forged" condition with widely varying tolerances on the shank and thus on the thread which is typically a cut thread. As is well known in the fastener art, the strength of a bolted connection can only be achieved if the bolt and nut threads are appropriately prestressed by application of a uniform bearing force exerted by the bolt head and nut. If the bolt head is not perpendicular with the bolt shank, a uniform stress cannot be imparted circumferentially to the threads resulting in a weaker connection than what is otherwise possible. For this reason, washers are typically speced by the AAR which, to some extent, may alleviate the problem.
Finally, the nut must, of course, be removably locked to the thread after the fastener has been appropriately torqued to it's prestressed, applied condition. The discussion concerns relatively large, standard sized, square nuts and bolt heads having side dimensions of about 2". Locking techniques conventionally used for small fasteners, such as a thread deformable plastic collar, are not applicable to a rail-switch splice application. Typically, a lock nut is supplied of two metal pieces with the second metal piece locked into the first metal piece and having an interference thread fit for locking purposes. The problem is simply that the insert becomes separated from the locked body for any number of reasons and the fastened connection loosens.
In general summary, the prior art has "improved" the forged bolt-nut assembly by making the nut a two-piece lock nut which has proved unacceptable and by simply upgrading the heat treatment and chemical properties of the bolt and nut to increase the physical properties of the fastener, specifically its yield or tensile strength. As briefly noted above, the threaded end portion of the bolt shank has threads cut thereon so that the outside diameter of the thread is maintained at the same diameter as the unthreaded portion of the bolt shank adjacent the bolt head. Because it is well known that cut threads are not as strong as rolled threads, one manufacturer supplies a bolt with a rolled thread at its terminal end, which, incidentally, is also supplied with a special torque breakaway appendage extending from the bolt head for application. When the threads are rolled onto the terminal end of the bolt blank, the outside diameter of the bolt shank increases to the specified bolt diameter, (i.e., 1", 11/8" , or 13/8" ). However, the unthreaded portion of the bolt shank adjacent the bolt head remains at its smaller pre-rolled blank diameter. Thus, the unthreaded shank portion of the bolt doesn't occupy the bolt opening through which it extends to the extent it should and produces a fundamentally inferior connection, resulting in looseness of the rail joint as well as a reduction in shear strength of the fastener.